gop, tea party favorite, he made his name taking on unions and now is possible a 2016 contender. On karl is back with the inside story of the man who was once the most divisive politician in america. Reporter: It was three years ago that the occupied movement was born right here in madison, wisconsin. The focus of the anger, republican governor scott walker for making a frontal assault against public employee unions, that would strip away most of their collective bargaining rights. Some have asked why we have to strip collective bargaining rights to balance the budget? The answer is simple the system is broken. Reporter: For a time, walker seemed to be the most hated man in wisconsin. Now, he's running for re-election and considered by many republicans to be a top contender for the 2016 presidential nominee. In a new book, he tells the story of the confrontation that brought 100,000 protesters outside his house in the capitol building for weeks on end. He was compared to hitler and osama bin laden. "Time" magazine declared him dead man walker. He and his family received death threats. There were signs out there that have my picture that have a stop sign on it. This tunnel connects from across the street. Reporter: The governor gave us a first-ever look at the secret tunnel he used to get in and out of his office during the occupation. One scene that makes the federal government shutdown tame in comparison -- when republicans rammed walker's bill through the assembly all hell broke loose. This was intense. You had not only protesters up there, but you had members screaming at members on the other side. Reporter: He acknowledged making mistakes along the way. But he's more critical of the way republicans in washington handle the government shutdown. Governor walker, for a while, you were the most divisive man in america. What did you learn from that? I came in wanting to fix things having a deficit. I was so eager to fix that, i didn't spend time talking about it with the people of this state. Reporter: When you were elected in 2010, you were some what of a tea party hero, what do you make of the tea party movement now? Tarnished in many ways. Blamed for the government shutdown. I don't think it's a movement that's myolithic. The way you make the compelling case to american people, the way we can make things better is by shutting things down. Reporter: As you know, the most unpopular are the republicans in congress. Where has the republican party just gone bad? Republicans at the state level are showing we're much more optimistic and we're speaking in terms that much more relevant. That's a real problem. Reporter: When walker talks about the type of candidate that the republicans should nominate in 2016. It sounds more than little that he's talking about himself. Describe for me the ideal republican presidential candidate in 2016. I think it's got to be an outsider, I think both the presidential and vice presidential nominees should be a former or current governor. Someone who has done successful things in their states. It rules out marco rubio and rand paul. All good guys. But it has to be someone companiesally removed from congress. Reporter: Your requirements also rule out paul ryan. Would you commit to the voters of wisconsin that you'll serve out a full second term? In my case, I have never made that commitment. Because to me it's not about the time you serve in office, I feel right now my calling is to be the governor of the state of wisconsin. Reporter: When you don't commit to serve a full second term. The door is open. You're certainly not ruling out. I don't rule anything out. Reporter: First thing first, he faces a re-election battle in wisconsin next year. For "this week," jonathan karl, abc news, madison, wisconsin.

This transcript has been automatically generated and may not be 100% accurate.